1. Field of the Invention
The invention relates to a ball screw apparatus.
2. Description of Related Art
There has been proposed a ball screw apparatus with a ball train interposed between a ball track in an outer periphery of a ball screw shaft and a ball track in an inner periphery of a ball nut. In the ball screw apparatus, a coil spring is interposed between a ball at an end of the ball train and a stopper pin fixed to the ball nut (see, for example, Published Japanese Translation of PCT Application No. 2010-505072 (JP 2010-505072 A)). In such a non-circulating ball screw apparatus, the coil spring does not contract when the ball screw shaft is rotationally driven with a low axial load imposed on the ball screw shaft. Thus, the balls in the ball train do not move relative to the ball nut. Therefore, the ball screw moves with the balls in the ball train sliding on the ball screw shaft.
On the other hand, when the ball screw is rotationally driven with a high axial load imposed on the ball screw shaft, the coil spring contracts to allow the ball screw to move with the balls rolling with respect to both the ball nut and the ball screw shaft. Thus, efficient screw power transmission is achieved.
The coil spring is housed in a raceway between the ball screw shaft and the ball nut. Thus, when the ball screw shaft is rotationally driven, the shape of the coil spring is likely to be distorted. Consequently, the coil spring may interfere with the ball crew shaft to cause wear or the like.